Growth, egg laying rate, and mortality of Biomphalaria glabrata (Portorican strain), maintained in groups of two, after exposure at different ages (1, 2, 4, 6, 8 weeks) to three miracidia each of Schistosoma mansoni (Liberian strain) were determined. The findings in infected snails were compared with those in exposed but uninfected (negative) and in unexposed snails. Younger snails were infected at higher rates, and these showed a significantly increased mortality. Young infected snails grew much slower than negative or unexposed snails; older infected snails grew at about the same rate as negative or control snails. A growth acceleration was not observed in parasitized animals. When prepatency to a high extent passed before sexual maturity, infected snails did not lay any eggs. In later infections the numbers of eggs dropped below those in healthy snails by the second week after exposure. Snails were dissected 15 weeks after exposure. Development of male reproductive organs (prostate gland, preputium plus penis sheath), in relation to shell diameter, was retarded particularly in animals infected before sexual maturity and highly retarded in growth. In larger animals the organs reached approximately their definitive size. Parasitized snails were partially deformed by an enlargement of their shell apertures on the right side.